The development of low cost-high performance radomes and sensor windows for millimeter wave applications is critical to successful implementation of present and future tactical missile systems. These weapon systems are expected to be operational over a wide domain of altitudes, ranges, and velocities. Successful implementation of the systems is often limited by current radome and sensor window technology as a result of their inability to handle the thermal environment and/or loading environments. Currently, slip cast fused silica and select glass-ceramics (Pyroceram, Rayceram, Barium Aluminosilicate) are the only ceramic materials in use or being considered for high performance radome applications due to their low cost and good electrical and thermal performance. However, use of these materials is limited due to the ever-increasing thermal, mechanical, and environmental requirements of advanced radome materials.
Present approaches to advanced ceramic radome and sensor window technology include reinforced slip cast fused silica and silicon nitride (Si.sub.3 N.sub.4) derivatives. Reinforced slip cast fused silica reportedly offers increased fracture toughness but at a cost to its electrical performance and ease of fabrication. Several different types of Si.sub.3 N.sub.4 based materials are under investigation but are thus far either operationally limited or prohibitively expensive due to fabrication processes.
Furthermore, in general, previous ceramic composite materials employ conventional ceramic whisker/platelet reinforcement, as opposed to the present inventions in-situ reinforcement. Conventional whisker/platelet reinforcement of ceramic materials consists of first manufacturing the ceramic whiskers or platelets. These whiskers/platelets are next mixed with matrix forming powders. The mixed powders and whiskers/platelets are then formed into a desired shape and densified by sintering.
There are many disadvantages to conventional whisker/platelet reinforcement. For example, the whiskers/platelets may become contaminated prior to incorporation into the matrix with these contaminants seriously impacting the ultimate performance of the composite. Toxicity concerns relate to physically handling the whiskers prior to their incorporation into the matrix and the disposal of the remaining whiskers thereafter. Platelets do not pose a toxicity problem. Difficulties in uniformly mixing the whiskers/platelets with the matrix forming powders occur because the whiskers/platelets routinely form clumps if not properly mixed which form anomalies in the fabricated composite. Mechanically intensive methods are commonly used to mix the whiskers/platelets and matrix forming powders but cause damage to the whiskers/platelets. The concentration of whiskers/platelets which can be added to the matrix forming powders and pressureless sintered to high theoretical densities is limited to less than 15% by volume. Higher whisker/platelet concentrations have been added by conventional means but require pressure-assisted sintering (hot pressing or hot isostatic sintering) resulting in increased manufacturing complexity and cost. Even with pressure-assisted sintering, whisker/platelet concentrations have been limited to less than 40% by volume. At higher whisker/platelet concentrations, high densities have not been achieved. When preparing a composite from whiskers/platelets mixed with matrix forming powders, the composite must be formed into the desired shape before (hot isostatic pressing) or during sintering (hot pressing). The latter approach requires extensive machining to achieve the desired final dimensions. During the forming of powders containing whiskers/platelets, the whiskers/platelet are preferentially aligned in directions dependent upon the shape forming flow fields. For example, during hot pressing, where the applied pressure is uniaxial, the whiskers/platelets are aligned into planes perpendicular to the pressing direction. A more drastic example is injection molding, where the whiskers/platelets take on preferred orientations which cause shape distortion during subsequent sintering accompanied by nonuniform shrinkage. In contrast to conventional whisker/platelet reinforcement, wherein the silicon nitride whiskers are grown outside of the matrix and thereafter added to the matrix powders, the present invention provides for the silicon nitride elongated fiber-like grains to be grown internally from a melt. This is what is meant by in-situ reinforcement.
There are many advantages to in-situ reinforcement. For example, contamination and toxicity are not a problem because the silicon nitride elongated fiber-like grains are not physically handled. The silicon nitride elongated fiber-like grains are grown internally, not grown externally and thereafter added. Further, by growing the silicon nitride elongated fiber-like grains internally, the difficulty of uniformly mixing the fiber-like grains with the matrix powders is obviated. Likewise, growing the silicon nitride elongated fiber-like grains internally allows for a composite that is totally isotropic and randomly reinforced, thus avoiding shape distortions during sintering that result from adding the whiskers externally. When the composite is totally isotropic and randomly reinforced, the electrical performance of the composite is greatly enhanced and the design of load bearing structure is simplified. Also, the present invention, by employing a low cost pressureless sintering method, achieves high densities (.gtoreq.97%) at high silicon nitride concentrations (50-90 vol. %).